1. Field of the Invention
The present invention relates to an improvement in a focusing device suitable for use in, for example, a camera.
2. Description of the related Art
Hitherto, an active range finding system has been known in which, as disclosed in Japanese Patent Laid-Open No. 60-59313, the quantity of light projected is determined based on the data obtained in previous range finding data such that the quantity of projected light is reduced when the previously obtained range finding data indicates that the distance to a subject is short, whereas, when the previously obtained range finding data indicates that the subject is at a long distance, the quantity of the projected light is increased. This arrangement is effective in preventing saturation of a signal processing circuit which otherwise may occur when the distance to the subject is small.
Meanwhile, Japanese Patent Laid-Open No. 1-123109 discloses an active type range finding system which calculates distance data through a double integration. In this device, the integration time is varied depending on the magnitude of the received light signal. More specifically, when the magnitude of the received light signal is small as in the case where the distance to a subject is large or in the case where the reflectivity of the subject is small, the integration time is set to a longer value, whereas, when the magnitude of the received light signal is large, the integration time is set to a shorter value. According to this arrangement, the S/N ratio is improved particularly when the distance to the subject is large and when the reflectivity of the subject is small, thus offering higher precision of the range finding operation.
These known range finding systems, however, have suffered from the following problems.
In the first-mentioned range finding system proposed in Japanese Patent Laid-Open No. 60-59313, the quantity of the light projected is reduced when the distance to the subject is small, and the range finding information is calculated only through a single range finding action with such a reduced quantity of projected light. Consequently, the output signal of the range finding system tends to be degraded by noise component, i.e., the S/N ratio tends to be lowered, resulting in an inferior range finding precision.
In the second-mentioned range finding system disclosed in Japanese Patent Laid-Open No. 1-123109, the integration time is set to a large value for a subject which is at a long distance or which has a small reflectivity, in order to improve the S/N ratio. The prolonged integration time tends to increase probability of erroneous measurement due to, for example, inclusion of a luminescent noise such as that produced by an electronic flash during the integrating operation. In addition, the projection efficiency (illuminance efficiency) of the light projection element tends to be reduced due to a long time of energization, with the result that the output of the light projection element is changed in relation to time. This appears as a difference in the light projection output between the period of ascending integration and the period of descending integration, thus impairing precision of the range finding operation.
In order to obviate these problems, the present applicant has proposed, in U.S. Pat. No. 5,107,449, a range finding system in which a plurality of cycles of range finding operation, each cycle being of a short time, are performed repeatedly, and range finding information is finally obtained by synthesizing the results of such a plurality of cycles of operation, thus achieving higher precision of range finding operation.
A problem encountered by this range finding system is that a considerable time is required until the final range finding information is obtained, due to repetition of many range finding cycles, so that the photographer may lose a photagraphic opportunity or mistime a shot.